Use each of the numbers 7, 8, 9, 10, and 11 once and only once to fill in the circles so that the sum of the numbers in the three horizontal circles equals the sum of the numbers in the three vertical circles.

Of the three possible solutions, which numbers can be used in the middle circle of the horizontal row? Show each of the resulting solutions.

So the numbers are 7,8,9,10 and 11. There are three horizontal circles and three verticle circles.

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I asked this question a few days ago, I still didn't understand, but the person helping me already left before responding again.